What I would love is a tool that lets me chart out timelines (ideally with a visual representation) for characters and story arcs. I want to be able to see how different characters interact over time (double points if it takes both time and location into account and let's me, somehow, see this visually, but I'd settle for just time too). I want to be able to roughly make up how a story arc interweaves with others and then flesh it out bit by bit. I want to see how characters are affected etc.

I've tried various other tools (diagramming, Gantt charts, brainstorming tools, note taking tools...) and they didn't really do it for me.

If such a tool existed for iPad (I've searched and come up blank), I would buy it in an instant (ok ok price dependent, but I'd be very interested). I'd almost build it myself, but.. who has the time?